After Singh's health allegedly deteoriated, his relatives admitted him at the Swarooprani Nehru hospital, SHO of Colonelganj police station, Alok Mishra said.
Police are currently interrogating him, the SHO said.
Yadav was yesterday shot at about 2.30am outside the Allahabad University's Tarachand hostel and died during treatment.
Yadav's wife, Monica, complained against Mukul Singh and other unidentified persons, after which an FIR was registered in the matter.
Senior Superintendent of Police Anand Kulkarni said police have retrieved the CCTV footage from the spot and identified four persons -- Anubhav, Akash, Avesh and Jagga -- who are absconding.
The picture will be more clear with the arrests of more persons, Kulkarni said.
Rajesh Kumar Yadav (40) had unsuccessfully contested the 2017 UP Assembly polls from Gyanpur seat in Bhadohi district.